Modulator employing trigger circuit



Dec. 14, 1948.. E. R. sHENK l-:TALv 2,456,026

MODULATOR *EMPLOYING TRIGGE CIRCUIT Filed June 14, 1947v s sheets-sheet 2 Z7 L1 y A fir/f ATTORNEY |NVENTOR5 Dec. 14, 1948. E. R. SHENK ETAL 2,456,026

I MODULATOR EMPLOYING TRIGGER CIRCUIT 4 Filed June 14, 1947 3 Sheets-Sheet 5 faam/nf: 5%5929.

Patented Dec. 14, 1948 MoDULAToR EMPLOYING TRIGGER CIRCUIT Eugene R. Shenk, Brooklyn, and Anthony Liguori,

New York, N. Y., assignors to Radio Corporation of America, a corporation-of Delaware Application June 14, 1947, Serial No. 754,758

14 Claims. (Cl. 179-1715) This application discloses a new and improved method of and means for modulating the amplitude of Wave energy of vcarrier wave frequency inaccordance with control potentials which may represent signals. It also -discloses improved means for linearly demodulating frequency modulated energy and for converting frequency modu-k lated energy-to frequencyand amplitudev modulated energy. y n,

In known forms of amplitude modulators such as those making useof multi-'grid tubes, the percentage of linear modulation is limited to approximately `35%. Aprimary purpose of .our invention is to provide modulation means` which operates linearly for a larger percentage modulation. In our modulator, we produce well over 90%,linear modulation. j

To attain this purpose, vwe use a trigger .or

locking circuit having one position of stability,

a differentiatingcircuit.for supplying pulse energy of carrier wave frequency yto the trigger circuit to flip the same Afronrits stable position at the carrier rate. A condenser is charged by a constant current source which is varied by the modulation and this condenser is discharged by the trigger circuit when it is. tripped out of its stable position. Thecharging current is a linear function of the modulation so that charging of the condenser is linear with respect'to the modu-l lation. The tripping rate and rate at which discharge of .the condenser vtakes placeare constant so that the charge'in the condenser varies with the modulation. `The voltage across the condenser, taken as output, is a' carrier almost fully modulated in amplitude by the control poten- 'tial or signal.

In the embodiment described, the constant current is suppliedby a pentode to the condenser wherein it is integrated.` By the term constant current here,l applicant means that the current is constant for each finite grid potenlated, is differentiated by a, network includingv R and C and then applied as pulses to the trigger circuit to re the same at a variable rate dependingon the repetitionrate ofthe frequency modulated energy. When the firing` isat a high rate, the condenser will havea smaller charge because the charging intervals areshort being measured 'by vintervals `between`- firing. s Whenv the ring rateis'low, `the condenser has `more charge. The output taken from the condenser will then be of varying amplitude as well as varying frequency. If this output is passed through a low pass filter Whose cut-off is just above the highest modulation frequency, then the original modulation may be recovered at ther filter output. Where frequency modulated energy is to be converted to correspondingly amplitude and 'frequency modulated energy, the output of the condenser is passed through a band pass filter which passes al1 components including th FM deviation band and its side bands.

It is believed that our invention as described above, will be understood by those versed in the art but details of various embodiments will now be described and in this description, reference will bemade to the attached drawings. In the drawings,

Fig. 1 illustrates the essential elements of a trigger circuit, means for applying carrier energy thereto to trip the same to discharge a condenser and means for modulating the charge in said condenser all in accordance with our invention,

Fig. 2 illustrates a modification of the embodiment of Fig. 1,

Fig. 3 shows by curves the forms of the voltage waves appearing at different points in the circuits of Figs. 1 and 2, f

Fig. 4 is a further modification of the arrangement of Fig. 1. Fig. 4 illustrates an embodiment of our improved demodulator or remodulator using the principles illustrated in Figs. 1 to 3 inclusive, and

Fig. 5 illustrates by curves the form of the waves appearing at different points in the embodiment of Fig. 4.

In Fig. 1, VI is the modulator tube, V5 and V6 are the tubes of the locking or triggering circuit.l The anode 6 of tube V5 is connected by resistor R5 to the positive terminal of a direct current source of potential Bl, the negative terminal of which is grounded being thereby connected to the cathode of tube V5 through the one-way valve in the form of a diode tube V4, and/or a similar tube V3 and resistor R4 in parallel with tube V3. The anode 8 of tube V6 is connected through resistor R8 shunted by condenser C4 to the grid I0 of tube V5. 'This anode direct current circuit then runs from the negative terminal of source B2 through the tube V6 resistor R8, resistor R6 in the grid circuit of tube V5 and back to ground and -l-BZ. Tube V6 has a grid resistor R7 and the control grids Illand l2 of tubes V5 and V6 are cross coupled there being a condenser C3 in the coupling between the anode 6 of tube V5 and the control grid I2 of tube V6. This is the triggering circuit having one condition of stability and at this condition of stability, tube V5 is cut-off while tube V6 draws heavy current to produce in resistor R5 a large potential drop applying the necessary high negative bias to the control grid l5 of tube V5 to cut this tube off.

The carrier wave is applied at terminals 28 and impressed by differentiating condenser Cl and resistor Rt on the cathode of a one-wave valve in the form of a diode tube V2 having its anode connected to the cathode of tube V5. The modu* lator tube Vl has its anode 22 connected to the point P2 at the upper end of condenser C2 which is the integrating condenser for the modulation current. The tube Vl is a pentode and has its control grid variably coupled by resistor Rl to` a modulating signal source at terminals 36. Its

cathode is connected by resistor R2 to the negative terminal of the source B2, the positive terminal of which is grounded. The screen grid is connected to a point on a potential divider including resistors REQ and Ri. The tube V'li, therefore, has no anode direct current circuit except such as is provided from B2 through resistor R2 through the impedance of tube Vl to point P2.

The tube Vl being a pentode has current ow which is constant with respect to grid bias i-rre- The tubes V5 and Vt constitute a trigger cir-` cuit operating in a well known manner. As ar.- ranged, tube V5 is cut oir by the negative potential on its grid lu due to heavy current owiirg in RB, through tube V5 which is fully conductive. This state of conductivity stands until an applied bias trips the locking circuit. Such a potential might be negative and applied to point Pl so that the potential on the cathode of tube V5 now approaches the potential on the grid l0 of tube V5 and heavy current ows through tube differentiated by the capacitor and resistor CI, R3 to -'Orm pulses-Jas Shown-vin .line A of Fig. 3. Diode tube V2 is poled andarranged -so that only the negative pulses are passed on to point Pl and to the cathode of tube V5. Every negative pulse passedpn to the cathode of tube V5 will rle'duce the positive potential on the cathode with respect to the grid l0 potential an amount sum 'cient to-'re the trigger circuit and cause tube V5 to conduct an'dvtube V6 to become non-conn ducting; that is, the trigger circuit is tripped out of its Aone stable condition.

Whenfthe tube V5 conducts, it discharges condenser C2; that is, the integrating condenser for` the current, the magnitude of which is controlled'bythe modulation. discharge path ici'V condenser "C2fi's from ground 'and the negative terminal of source B-If, condenser C2, tube V3 (and a slight amount through resistor R),

V5 to cut oir tube V5 by applying a more negative pulse to its grid l2 through condenser C3. Tube V@ then remains cut-off for a time measured by the values of capacitor C3 and resistor R'i. That is, during the time it takes the negative charge on C3 to leak off through resistor R1.

The tube V5 then` begins to draw current toY apply a more negative potential to the `control grid of tube V5 and the action is accumulative as is well known to trip the triggering circuit back to its one stable condition Whereat tube V5 is fully out off and tube V5 is fully conductive. Thearrangement here is` such that the bias at point Pl, in the absence of carrier energy applied at Cl, causes the triggering circuit toreturn to this one condition of stability inthe event it is tripped therefrom.

Tube Vl has modulation applied to its grid and capacitor C2 is charged by current ow from ground and the negative terminal of source B2 to the positive terminal of source B2 through resistor R2, tube Vl and into condenser C2' to place a negative potential at the point lP2 and on that side of condenser C2. This negative potential, however, is never sufficient to act, through one-Way tube V3, on the cathode of tube V5 a't point Pl to trip the trigger circuit out of its one condition of stability.

The carrierl consists of square waves which are constan-t current to condenser C2.

tube V5,.resistor R5 to +BI and back to ground and BL lThe tube v5 conducts roiat least is long "as the time requiredl Lto discharge 'capacitor o`2. The active time of the trigger circuit, which,

as shown above, is determined primarily by a time constant (capacitor C3 and' resistor R1) is slightly greater; than the time required to discharge capacitor C2 vand after condenser C2 is muy discharged, the one-wey valve diode vs 30 ,supplies the additional 'current'ih the direct current output path of tubev V5 t o keep the tube V5 conducting. The resistor '124r` is any isolating resistor between the cathode vof ttubeV and capacito'r C2 which `prevents 'the pulses at Pl' 'from Yaffecting.unduly the triangular Awaves `a`t P2. The tube V3 'supplies' :a low impedance path for the erections from capacitor -oz 'ang tube v4 to reach the output impedance of tube V5.

A summary of the operation will now be given eventhoughfitfis thought the same will be clear from the foregoing description.

Constant current from tube Vil is-'integrated on condenser IC2 causing thevolt'age across C2 to vary in .proportion to they magnitude ofthe current and Y,the time that it `iiows. 'C2 'is yofjsucl'i Value that the current iiovv from tube Viy never` causes the voltage across 'C2 to. be large enough to nre the trigger circuit. IY-he firing l'salways done by the carrier pulses from `tube V2 and r'ecurs at a constant rate. vThe modulating signal applied to the grid of tube VI will cause Ithe current through tube Vl to vary. The current vhow 'from tube V-l to 4condenser C2 is conjstantmfor anyI one value of grid signal voltage on tubeV'l 'despite the variations in plate voltage oftubeVl, i. e.,"ti'1be Vil supplies a n'y lin-ear increase 'or decrease in signa'lvoltage results in a linear increase `or decreasev in current flow-into condenser C2. ps in'c'e the trigger i-sfre'd' at regular intervals, (C 2` is dischargedfat regular Vintervals) the amplitude `ofi'thevoltage across condenser C2 isadir'e'c'tiunc on--ofthernagnitude of the 'Current howf'rem tube-v1, which is a direct function or the sig-nal veitagetntrie rgridef tube Vl. The wave appearing fat p'oint Pl and on the cathode of tubeVSwhichfres the trigger circuit y pearing at point PI varies.

across condenserCZA which is a linearly'increasing f charge and a very short discharge time. `The.

carrier frequency and side bands but attenuatesv all modulating signal yfrequencies and all harmonies of the carrier frequency. Theoutput will then be a sine wave, the amplitude of which varies at signal frequency, and willhave a Wave form as shown in line E of Fig. 3. Then 'an arrangement as shown in Fig. 2 may be used. This embodiment is similar to that of Fig. l except as follows. An amplifier tube Vl has its'control gridv The tube V1 has its' 36 coupled to the-point P2. cathode grounded through acathode resistor R and its grid 35 is coupled to ground by a'grid return resistorRS.- The anode direct current circuit includes a load resistor RH and this loadresi-stor supplies output to'a band pass lter '50;' The 'triangular waves appearing at the ypoint P2 are amplified ina the tube Vly and fed to'theband pass filter 6B. The band pass filter will pass the carrier frequency and the' side 'bands but willfattenuate allfmodulating signal frequencies and all harmonics 'of the carrier frequency. The output willbe varying amplitude sine 'waves of a carrier frequency corresponding toA vthe free quency of the pulse energy impressed at the input 2li. The forni of the outputf'is shown in lin-e E of When the circuit is to be used as a de-modui lator or 're-modulator, it is modied asillustrated inFig. 4. The arrangement of Fig. 4 is similar`v` to the arrangementofFig. 1 except in the 'following respects. In Fig. 4,' the signal appliedvat the leads 20 is frequency modulated square Waves.A

This frequency modulated square wave energy is fed, as in Fig'. l, over a differentiatingnetwork Cl, R3 which forms pulses as shown inline A of Fig. 5. These pulses are passed to a one-Way valve tube to the cathode of 'the trigger circuit tube V5. No signal is'fed tothe control gridof tube V l. On the contrary, thistuoe Vl is of fixed gain adjusted as desired to supply the desired charging current to point P2 and condenser Cl. By keeping the potential betweenv the grid and cathode of tube Vl, constant, the charging current is maintained constant irrespectivefof varia'- `tive pulse is passed by one-way valve tube V2 to the cathodeof trigger ytube V5 at point Pl. The potential at point PI isas illustrated in line B of Fig. 5. rlhe voltage developed across C2 and Pl is proportional to 'T and inversely proportional to F. Each negative pulse will re the trigger circuit as desired above. Due to the varying frequency ofthe carrier impressed atf2`, the time interval between these negative pulses ap- Since the charging current from tube Vl to capacitor C2 is of xed value, the voltage across capacitor C2 is propor-v are of short duration.

` rier.

6 tional to time. thus fires the trigger circuit at Varying vintervals depending on its frequency. quenciesare high, the intervals between rings The tube Vl then has less chance to charge up capacitor C2 and the same is not charged very much'.v When the modulation frequency of the carrier at 20 is low 'the intervals between flringsof the trigger circuits are longerk and capacitor C2 is 'charged to a higher negative voltage. The output at the point'PZ is of triangular wave form and of varying amplitude and varying frequency. This output then has a 'wave form as illustrated in line C of Fig.

5. Line 3'shoWs the Voltage across C2. At fre-r quency F, the voltage across C2 reaches a'value 2E,'Where T, and i At frequency 2F thevoltage across C2 reaches Aand at frequency 3F the voltage across C2 reaches chosen to make the relationship ofthe output D.C. voltages for the different frequencies read-f ily discernible. The amplitudeof this Wave is inversely proportional to the frequency of the fre-` quency modulated carrier. at the input 20. The amplitude is proportional to 4time T which is The frequency of this triangularwave is the same as the frequency of the carrier supplied at 20.

`Now, ifv a low .pass filter is connected to the load resistorof tube Vrand arranged to. pass only the Vsignal modulating the frequency of the carrier'impressed at 20, the loutput of the filter will be the original signal used to modulate the car- The vsignal is then represented by line D of Fig, 5. The highest modulation frequency must, of course, be lower in frequency than thelowest carrier frequency.

Where itis desired to convert the frequency rmodulated carrier at input leads 20 to a carrier which is correspondingly amplitude and frequency modulated, a band pass filter may be connected tothe plate of tube Vl. This kfilter is then arranged to passthe entire frequency modulated wave band comprising the carrier and both of its side bands. The entire spectrumis passedby the band pass filter 60. The filter 69 attenuates or excludes the modulation frequencies and harmonics of the carrier. The' output will then consist of varying frequency, sine waves of Varying amplitude as illustrated in line E of Fig. 5.' The amplitude of these waves is proportional 'to T, which is f 4What is claimed is;

l. In a signalling apparatus, a trigger circuit comprising two tubes having electrodes and connections such that the circuithas one condition of stability to which it returns at a predetermined time interval after it is tripped therefrom, a capacitor, a discharging path for the capacitor including the impedance between the electrodes of one of said tubes, a charging path for said ca- The frequency modulated lsignal` Where' the free' ancegoac 7 pacitor including:a -sourceof continuous unidirec tional current, connections 'tosaid trigger circuit fortrippingthesame .from said one condition of stability :at aqrecurring rate andan output circuit .coupled to saidrcapacitor.

2. Inv a signalling apparatus, -a trigger circuit comprising. two vtubes havingeiectrodes and connectionsv suchr that the `'circuit vhas one condition of stability'to which itreturns ata predetermined timeeinteryal after it is ytripped therefrom, acapacitor, :a -discharging ypath for the -capacitor including the impedance between the electrodes of one `of isaid tubes, a charging pathfor said capacitorincluding a source of continuous unidirectional current the magnitude of which varies with signals, a source .of alternating voltage of constant frequency connected to said trigger circuit for tripping the same from said one condition ofstabilityat a constant rate'and anloutput circuit 'coupled to said capacitor.

3. Apparatus as recited in'claim 2wherein said output circuit is coupledv .to said capacitor by a lter circuit.

4. Apparatus as recited in claim 3 wherein said filter- -is ofthe band pass type with the band pass substantially centered on `the frequency oi said alternating voltage of constant frequency,

5. In a signalling apparatus, a trigger circuit comprising two tubes having electrodes and connections 'such that `the vcircuit has one condition of stability towhich'itv returnsat a predetermined time interval after Yit is tripped therefrom, aV capacitor., a discharging path for the capacitor including the impedance between the electrodes of one of said tubes, a charging `path for said capacitor including a source of continuous unidirectional current of constant magnitude, a source of alternating voltage of a frequency which variesinaccordance with signals coupled to said trigger .circuit for tripping the same from said one condition of `stability at' a varying rate and an output -circuit coupled to said capacitor.

6. Apparatus as'recitedin claim "5 wherein said output circuit nis coupled to said `capacitor by a filter circuit;

7. Apparatus-as recited in 'claim 6 wherein said filter is of the low pass type with its cut-off above the highest signal frequency.

8. Apparatus as recited in claim 6 wherein said filter is oi the band pass type with its band pass substantially centered on the frequency of said alternating voltage.

"9. `In a signalling apparatus, a trigger circuit comprising two tubes-having electrodes and connections such'that ithas'one condition of Astability to fwhichitreturns at a predetermined time interval after it is tripped therefrom, a condenser, a discharging path therefor including the impedancebetween :electrodes of one `of said tubes, `a third tube'having an anode, a cathode and agrid operated so that the anode current flow in the tube is linearly related to `its grid potential irrespective of ranode -potential variations, a charging path for said condenser including the impedance between the anode and cathode of the third tube, connections to said trigger circuit for tripping the same at a rate such that the time intervals between tripping operation are greater 'than said first time vintervals and anoutput circuit coupled to saidcondenser.

I0. Ine'a signalling apparatus, a trigger circuit comprising two tubes having electrodesand con- 8 nections :suchithatithas one .condition vo'fstability to which it freturnsata predetermined time intervalafter itristripped therefrom, acondensena discharging paththerefor including the impedancebetweenzelectrodes of one of :said tubes, 'a

third-tube` having' an anode, a: cathode and a grid operated so that theanode current flow in the tube is linearly related to its grid potential irrespecti-Ve of anode potential variations, a charging pathfor said .condenser'including theimpedance betweenthe anode: and cathode .of thethird tube, connections to the electrode of said third tube torrnodulate'thepotential'therein in accordance with signals, connections to said trigger circuit for tripping 4the same at a rate such that the time intervals between tripping operation is greater than saidrsttime intervals and an out.- put circuit coupledto said condenser.

11. Apparatus as recited inclaim l0 wherein a nlterrv circuit is in the coupling to'said output circuit.

12. yin a signalling-apparatus, a trigger circuit comprising twotubes having electrodes and connections such that it has one condition of stability'to'which` it returns at a predetermined time interval :after it is trippedftherefrom, av condenser, a discharging `path therefor includingthe impedance between electrodes of one of said tubes, a thirdtubie having an anode, a cathode and a grid operated so that the anode current flow in the tube is linearly `related to its grid potential irrespective of "anode potential variations, a charging path `for said vcondenser including the impedance betweenthe anode and cathode of the third tub'e, a source of voltage the frequency of which varies irl-.accordance 4with signals from said.

source, connections to said trigger circuit for ltripping the same at arate fsuch that the time intervals between tripping operation is greater than said first-time intervalsvand an -output circuit connected tosaid condenser.

V13. Apparatus-as recited in claim 12 including a filter circuit in said'last named coupling.

14, In a signalling apparatus, a trigger comprising two tubes having electrodesand connections such that it has one condition of stability to which it returns at -a predetermined time interval` after'it is tripped therefrom, a condenser and a discharging path therefor including one of said tubes, a third tube having an anode, a cathode and a grid operated so that thefcurrent flow in the tube is linearly related to its grid potential irrespective Vof anode potential variations, a-chargingpath for said condenser including the impedance between the lanode and cathodev ofthe third tube, a network for modifying the shape of recurring `potentialsof carrier Afrequency connected to Isaid trigger circuit for firing the same at said recurring rate and anoutput circuit connected to said condenser.

EUGENE R. S-IENK. ANTHONY LGUORI.

REFERENCES CITED The following references are of Vrecord in the.

le of this patent:

UNITED STATES PATENTS Number Name Date 2,262,838 Deloraineet al Nov. 18, 1941 2,416,328 Labin Feb. 25, 194.7

Disclaimer 2,456,026.-Eugene R. Shen/c, Brooklyn', andf-Anthony Lz'goum', New York, N. Y.

ODULATOR EMPLOYING TRIGGER CiRCUIT. P

Dislaimer led July 28, 19,15), th Hereby nters this disclaimer toclaims l [Oicial Gazette, Sept. 5, 1.950.]

e assignee, R

wenn dated' Dec. 14, 194s.

adfio Corporation of Amrica. i 1, 2, 9, 10, and 14 of said patent. 

